With the development of multimedia information technology, optical technology-based touch screens have been applied more and more widely. FIG. 1 is a schematic structure diagram of an optical touch screen in the prior art. As shown in FIG. 1, the optical touch screen comprises a frame 1100, a touch panel 1200, a retro-reflection strip 1300, two cameras 1400 as image sensors and two light sources 1500. The frame 1100 comprises a first edge 1101, a second edge 1102, a third edge 1103 and a fourth edge 1104. The touch panel 1200 is surrounded by the frame 1100. The retro-reflection strip 1300 is fixed on the second edge 1102, the third edge 1103 and the fourth edge 1104 of the frame 1100. The two cameras 1400 are respectively fixed at the corner between the first edge 1101 and the second edge 1102 and the corner between the first edge 1101 and the fourth edge 1104 of the frame 1100, while the two light sources 1500 are respectively fixed upon the two cameras 1400.
There exist the following defects for the optical touch screen in FIG. 1:
Firstly, there exists an invalid touch area in the vicinity of the common edge (i.e. the edge 1101) between the two corners where the two cameras 1400 are fixed. FIG. 2 is a schematic diagram of the invalid touch area in the touch screen illustrated by FIG. 1. As shown in FIG. 2, when a touch object moves from P1 to P2, although the shift distance is large in horizontal direction, the shift distance of the shade formed by the touch object on the retro-reflection strip 1300 is very small, thus, it is impossible to accurately determine the positions of P1 and P2 according to the position of the shade. Moreover, if there are two touch objects P1 and P2 on the optical touch screen, namely, when a multi-touch occurs, although the distance between the two touch objects P1 and P2 is large on the touch panel, the distance between the shades respectively formed by the two objects on the retro-reflection strip 1300 is quite small, so it is impossible to accurately determine the positions of P1 and P2 according to the positions of the shades either.
Secondly, when two or more touch objects conduct touch operations on the touch screen, false touch points will occur (or called “ghost points”). FIG. 3 shows a schematic diagram of the false touch points on the touch screen illustrated by FIG. 1. As shown in FIG. 3, T1 and T2 are real touch points of two touch objects, however, according to the triangulation method, four touch points T1, T2, G1 and G2 of these two touch objects would be obtained, wherein G1 and G2 are the false touch points. In this case, the real touch positions of the multiple touch objects cannot be determined.